media: vimc: remove the function vimc_unregister

The function vimc_unregister is called only from
one place in the code and has only 3 lines so it
has no justification.

Signed-off-by: Dafna Hirschfeld <dafna.hirschfeld@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>